freopen

Changes the file associated with stdout to a specified file.

void freopen(const char pathname, ...)

The freopen function is typically a data recording function that changes the file that's associated with stdout to the file that's specified by the arguments in printf fashion.

If the "" string is used, the output is again restored to stdout.

The freopen function isn't only data-recording but also destructive, because you can use it to overwrite arbitrary files.

Example 8-17 How to use freopen to write to a specified file and log a system call

The script opens with a pragma to enable destructive functions in DTrace. You can alternatively remove this line and run the script with dtrace -w. The freopen function is destructive because it writes to a file on the file system and can overwrite existing files. The example creates a temporary log file to track the process names that make a mkdir system call while the program is running.

 #pragma D option destructive
 dtrace:::BEGIN
 {
        freopen("/tmp/dlog");
 
 }
 syscall:vmlinux:mkdir:entry
 {
        printf("%Y-> %s \n",walltimestamp,execname);
 }